Css is a browser side technology like javascript. It is not related to any server setting and it does not matter which controlpanel, operating system, webserver etc. you use to deliver the web pages, so this is not related to ispconfig in any way.

Thanks both for your help!
And yes i realize that Till, but do you have an explanation then?
I mean, when i visit the html file on the first server i can see the border-radius and on the ISPConfig3 server not???
Same pc, same browser, same html file, different servers (but both Debian).

Might be a browser extension or any setting on your windows workstation that e.g. handles that one site is a trusted system and the oter one not or you have any browser extension installed that loades a different rendering engine for one IP etc.

Another option might be that the file is not the same or that the file has any relative links or includes to css or javascript or images that are missing.

The issue with the rounded corners fix for IE7/8 is solved. The htc file was not loaded correctly. The path to the htc file was incorrect. After fixing this we discovered another issue. The htc solution will only fix the rounded borders issue for all borders, but not if you only want to change the radius of one or two borders. Thank you for trying to help us out!